This invention relates to a signal receiving circuit in optical communication, and is intended to prevent an erroneous operation in signal reception when an optical input is not available at all.
FIG. 1 shows an optical communication system using transceivers each of which is obtained by combining a transmitter and a receiver together. In FIG. 1, reference numerals 1 and 2 designate transceivers which are obtained by combining signal receiving circuits 1a and 2a and signal transmitting circuits 1b and 2b, respectively. Data communication using two values "0" and "1" for instance, is carried out between the transceivers 1 and 2 in the manner to be described.
Data corresponding to the value "0" is communicated by transmitting light which is modulated with a frequency f.sub.1, and data corresponding to the value "1" is communicated by transmitting light which is modulated with a frequency f.sub.2.
In a conventional signal receiving circuit, as shown in FIG. 2, an optical input which is modulated with the frequencies f.sub.1 and f.sub.2 is converted into an electrical signal by a light receiving element, 3 and the electrical signal thus obtained is amplified by an amplifier 4 and are converted into binary data by a demodulator 5. In this case, where the gain of the amplifier 4 has been increased in order to receive even a small optical input, the amplifier 4 outputs random signals owing to noise or the like even when no optical input is available at all because of a failure in the signal transmitting system. Therefore, depending on the demodulator, random data are outputted, thus causing an erroneous operation in signal reception.
Furthermore if the gain of the amplifier 4 is decreases in order to prevent the aforementioned erroneous operation, it is impossible to receive a small optical input.
When no optical input is received at all, it should be determined that a trouble occurs in the signal transmitting system, and it is necessary to provide an identification signal therefor, which is difficult.